1. Field of the Invention
The present invention relates to a positively-charging toner for a two-component-type magnetic developing agent having excellent spent resistance. More specifically, the invention relates to a toner which contains no charge control agent, does not scatter during the developing, enables the image to be efficiently transferred, makes it possible to form an image of a high density, and helps extend the life of the toner and the carrier.
2. Description of Prior Art
A so-called two-component-type magnetic developing agent has been extensively used for developing electrostatic charge image formed on an electrophotosensitive material.
The two-component-type magnetic developing agent comprises a composition of a magnetic carrier of an iron powder or ferrite grains and an electroscopic toner composed of a coloring resin composition. To carry out the developing, the magnetic carrier and the toner are mixed together to electrically charge the toner grains to a predetermined polarity, the mixture is carried to the photosensitive material in the form of a magnetic brush, the surface of the photosensitive material is rubbed by the magnetic brush, and the electrically charged toner is adsorbed and held by the charge image on the surface of the photosensitive material to form a visible image.
A charge control agent is usually contained in the toner grains in order to control the polarity of the toner grains by frictional charging. A negative charge control agent such as a metal-containing complex salt dyestuff or a metal complex of oxycarboxylic acid is used for the negatively-charging toner (e.g., see Japanese Laid-Open Patent Publication No. 67268/1991), and a positive charge control agent such as an oil-soluble dyestuff like Nigrosine or an amine control agent is used for the positively-charging toner (e.g., see Japanese Laid-Open Patent Publication No. 106249/1981).
It has long been known to use a magnetic toner as a toner for the two-component-type magnetic developing agent. For instance, the above Japanese Laid-Open Patent Publication No. 106249/1981 and Japanese Laid-Open Patent Publication No. 162563/1984 disclose a magnetic powder-containing toner which contains a magnetic powder therein. The above Japanese Laid-Open Patent Publication No. 67268/1991 discloses a magnetic powder-carrying toner obtained by adding and mixing a silica powder and a magnetic powder to the toner.
It has been known that the two-component-type magnetic developing agent exhibits satisfactory electrical charging performance in an initial state of when the magnetic carrier and the toner are used being mixed together but loses its charging performance due to the formation of a so-called spent (toner) and its life is shortened.
The spent (toner) is a phenomenon in which the toner component adheres and precipitates like a film on the surface of the magnetic carrier. Since the surface of the magnetic carrier becomes close to that of the toner, the triboelectricity approach each other making it difficult: to obtain a desired charging performance. When the spent is formed, therefore, the magnetic carrier must be replaced by a new one.